1. Field of the Invention
The present invention generally relates to driving motors for use in, for example, floppy disk drives, and more particularly, to a motor provided with a detector for detecting the rotational speed of a rotor.
2. Description of the Related Art
FIG. 6 partially illustrates the internal structure of a typical known motor, and more specifically, illustrates the relationship between a driving slot unit 20 and a FG pattern 30, which is provided for detecting the rotational speed of a motor. The slot unit 20 is formed by winding driving coils 7 around rod-like iron-core yokes 6. A total of twelve slots are radially provided at a regular pitch having an angle of 30.degree. and are fixed on a stator substrate. In the FG pattern 30, radially extending detecting lines 13 are formed in a zigzag shape, and a line 31 is extended on the outer side of the detecting lines 13 in the circumferential direction. One end of the FG pattern 30 is connected to a ground, and one end of the line 31a serves as an output portion.
Referring to FIG. 1, a disk-like rotor 3 is rotatably provided on the FG pattern 30 (12 in FIG. 1), and a ring-like rotor magnet 8 is fixed on the peripheral portion of the rotor 3. Formed on the surface of the rotor magnet 8 facing the FG pattern 30 is a rotational-speed detecting magnetized portion 10 in which N poles and S poles are alternately magnetized at the same pitch as that of the detecting lines 13. Also provided on the surface of the rotor magnet 8 facing the forward end of the slot unit 20 is a driving magnetized portion 9 in which N poles and S poles are alternately formed at a pitch wider than that of the rotational-speed detecting magnetized portion 10.
The motor illustrated in FIG. 6 is a three-phase motor. Upon supplying U-, V-, and W-phase alternating currents having a 120.degree. phase difference to the driving coils 7 of the slot unit 20, the rotor magnet 8 is rotated in a single direction (clockwise in FIG. 6) due to the relationship between the iron yokes 6 magnetized by the driving coils 7 and the magnetic poles of the driving magnetized portion 9. Then, the rotational-speed detecting magnetized portion 10 also rotates over the FG pattern 30 to generate a current in the detecting lines 13 due to a change in the magnetic flux, thereby obtaining a frequency generating (FG) signal from the above-described output portion 31a. Based on this FG signal, the frequency of a driving current to be supplied to each driving coil 7 is adjusted, thereby controlling the rotational speed of the motor.
FIG. 7 schematically illustrates the flow of a current in the known FG pattern 30. The FG pattern 30 has detecting lines 13 and an outer peripheral line 13a and an inner peripheral line 13b for connecting adjacent detecting lines 13. As discussed above, the FG pattern 30 generates a current based on a change in the magnetic flux due to the rotation of the rotational-speed detecting magnetized portion 10 over the FG pattern 30, thereby obtaining a FG signal. In practice, however, since the slot unit 20 and the FG pattern 30 are placed in proximity with each other, noise is disadvantageously superimposed on the FG pattern 30 from the magnetic flux of the slot unit 20.
A driving alternating current is supplied to each driving coil 7, and the magnetic field of the slot unit 20 is altered in response to a change in the amplitude of the alternating current. A change in the magnetic field induces a current in the FG pattern 30, thereby superimposing noise on a FG signal. Among the unwanted currents generated in the FG pattern 30, the direction of a current a01 is reverse to that of a current a02 in the FG pattern 30, thereby being canceled by the other. In the lines 13a and 13b, however, currents a1, a2, a3, a4, . . . indicated by the white arrows in FIG. 7, all of which are oriented in the same direction in the FG pattern 30, are momentarily generated, thereby appearing as noise. On the other hand, in the example shown in FIG. 7, a current c indicated by the black arrow is momentarily generated in the line 31 extending outside the detecting lines 13 due to a change in the magnetic field of the slot unit 20. The direction of the current c is reverse to that of the added current (a1+a2+a3+a4+. . . ) generated in the lines 13a and 13b, and the currents are thus offset. In this manner, hitherto, by providing the additional line 31 on the outer side of the detecting lines 13 for the FG pattern 30, the superimposition of noise on a FG signal caused by a magnetic field of the slot unit 20 is prevented.
In the example illustrated in FIG. 7, however, it is necessary to form the additional line 31 on the outer side of the detecting lines 13 and the outer and inner lines 13a and 13b, which are formed in a zigzag shape on the whole. This inevitably increases the diameter of the FG pattern 30 and accordingly enlarges the overall motor. Additionally, the wiring of the FG pattern 30 becomes complicated, thereby increasing the manufacturing cost of the FG pattern 30. Thus, the miniaturization of the motor, which has been increasingly demanded according to recent trends, is hampered.
FIG. 7 also reveals that the line 31 is positioned farther away from the slot unit 20 than the lines 13a and 13b of the FG pattern 30. Accordingly, the current generated in the line 31 becomes smaller than the current induced in the lines 13a and 13b, thereby failing to completely eliminate the unwanted currents, which appear as noise in the FG pattern 30.